WO2018138261A1

WO2018138261A1 - Use of a lipoic acid ester as an anti-wear additive and/or friction modifier

Info

Publication number: WO2018138261A1
Application number: PCT/EP2018/051959
Authority: WO
Inventors: Keihann Yavari; Karima ZITOUNI
Original assignee: Oleon Nv
Priority date: 2017-01-27
Filing date: 2018-01-26
Publication date: 2018-08-02
Also published as: FR3062392A1

Abstract

The invention relates to the use of a lipoic acid ester as an anti-wear additive and/or friction modifier, and to the compositions comprising this type of additive.

Description

USE OF A LIPOIC ACID ESTER AS ANTI-WEAR ADDITIVE AND / OR FRICTION MODIFIER

The present invention relates to the use of a certain type of ester as an antiwear additive and / or friction modifier, compositions comprising it, and their method of preparation.

The anti-wear and friction modifier additives are particularly used in the field of lubricants.

A lubricating composition generally comprises an oil, usually the major constituent (the constituent whose content is the highest), and one or more additive (s).

An additive is used to enhance one or more intrinsic property (s) of the oil and / or provide it with one or more additional property (s).

An anti-wear additive reduces the wear of parts, such as metal parts, when they come into contact.

Among the anti-wear additives include polysulfides, dithiocarbamates, chlorinated paraffins, phosphates, and thio-phosphates. Zinc dialkyl dithiophosphate (ZDDP) is one of the most used additives because it is particularly effective as anti-wear. However, this additive has a high toxicity. The decomposition of the ZDDP releases various sulfur gases, such as hydrogen sulfide (H ₂ S) and mercaptans.

There is therefore still a need for renewable, environmentally friendly and effective anti-wear additives with mineral oil.

A friction modifying additive (or friction reducing agent) reduces the friction of parts, such as metal parts, when they come into contact.

Among the most commonly used friction modifying additives, esters such as glycerol mono oleate (GMO), certain fatty acids and amides such as oleate diethanolamide (ODEA) can be mentioned.

The publication "Lipoate ester multifunctional lubricant additives"; Biresaw et al .; Industrial & Engineering Chemistry Research (2016), 55, 373-383, describes monoesters of renewable origin and their use as a multifunctional additive in a vegetable oil. In particular, they are lipoic acid esters and C4-C18 monoalcohols. The authors show that these purified esters are additives making it possible to improve the performance of an oleic sunflower oil, such as its viscosity index, its antioxidant properties and extreme pressure. On the other hand, these monolipoates of monoalcohol do not bring any anti-wear properties and properties modifying friction with this vegetable oil. In fact, oleic sunflower oil additive with 5% by weight of monolipoate monoalcohol has coefficients of friction and results in terms of wear equivalent or greater than those obtained with non-additive oleic sunflower oil (see Table 8 of this publication).

However, the work of the inventors has made it possible to demonstrate that, unexpectedly, these same esters make it possible to improve the anti-wear property as well as the friction modifying property of a mineral oil.

The invention therefore relates to the use of a lipoic acid ester as an antiwear additive and / or friction modifier for mineral oil.

Lipoic acid esters have extreme pressure, antioxidant and / or viscosity index improver properties.

The invention therefore also relates to the use of a lipoic acid ester as an anti-wear additive and / or friction modifier for mineral oil, and as an extreme pressure additive, antioxidant additive and / or or additive improving the viscosity index.

Lipoic acid is also called 5- (1,2-dithiolan-3-yl) pentanoic acid, 6,8-dithiooctanoic acid or thioctic acid.

Mineral oils are oils from petroleum refining. They consist essentially of carbon and hydrogen atoms, such as paraffinic oils, hydrorefined oils, hydrocracked oils and hydroisomerized oils.

Mineral oils are categorized into three groups:

Group I oils: these oils have a saturated hydrocarbon content of less than 90% by mass, an aromatic hydrocarbon content of greater than 1.7% by mass, a sulfur content of greater than 0.03% by weight, and a viscosity number of between 80 and 120;

group II oils: these oils have a saturated hydrocarbon content of greater than 90% by mass, an aromatic hydrocarbon content of less than 1% by mass, a sulfur content of less than 0.03% by mass, and a viscosity number of between 80 and 120;

group III oils: these oils have a saturated hydrocarbon content content of greater than 90% by mass, an aromatic hydrocarbon content of less than 1 7% by mass, a content in sulfur less than 0.03% by weight, and a viscosity number of greater than 120;

the percentages by mass being given in relation to the mass of the oil. It should be noted that, in the context of the present application, and unless otherwise stipulated, the ranges of values indicated are inclusive.

More particularly, the lipoic acid ester is represented by the following formula (I):

n being an integer from 1 to 8, representing the number of substituents L on R;

and wherein R is an acyclic, linear or branched hydrocarbon chain, optionally substituted with oxygen atoms, i.e., a chain consisting of carbon, hydrogen, and optionally one or more atom (s) of oxygen.

Preferably, the lipoic acid ester comprises at least 16 carbon atoms, preferably at least 24 carbon atoms.

The lipoic acid ester may be a mixture of several lipoic acid esters.

The lipoic acid esters are obtainable by esterification of lipoic acid with an alcohol or transesterification of lipoic acid with a triglyceride or a carboxylic acid ester other than a lipoic acid ester.

Preferably, the lipoic acid esters are obtainable by esterification of lipoic acid with an alcohol.

Esterification may be carried out according to any known method, in particular by chemical or enzymatic means. The esterification may for example be carried out by heating the reagents, lipoic acid and alcohol, at a temperature of at least 90 ° C, in the presence or absence of catalyst. Esterification can also be carried out using an enzyme by heating at 70 ° C.

By alcohol is meant an acyclic hydrocarbon chain, linear or branched, substituted only by one or more hydroxyl function (s) and optionally having one or more oxygen atom (s). There are no other heteroatoms or substitutions in the alcohol.

As alcohols containing a hydroxyl function, also called monoalcohols, mention may be made of methanol, ethanol, isobutanol, heptan-1-ol, octan-1-ol, 2-ethylhexanol and 7-methyl octan-1-ol, 6-methylpentan-1-ol, nonanol (or nonan-1-ol or pelargonic alcohol), decan-1-ol, dodecan-1-ol and octadecan-1-ol.

As alcohols having two hydroxyl functions, also called diols, mention may be made of 1,2-ethanediol or ethylene glycol, 1,6-hexanediol, 1,2-propanediol or propylene glycol, 1,3-propanediol, 1 , 4-butanediol, 2-butyl-2-ethyl-1,3-propanediol, 2-methyl-1,3-propanediol, neopentyl glycol, diol dimers, polyethylene glycol such as diethylene glycol and triethylene glycol .

As alcohols comprising three hydroxyl functions, also called triols, mention may be made of glycerol and trimethylolpropane.

As an alcohol having more than three hydroxyl functions, mention may be made of pentaerythritol, di-pentaerythritol and polyglycerols.

Preferably, the alcohol has one, two or three hydroxyl functions. More preferably, the alcohol has one or two hydroxyl function (s), that is, the alcohol is a monoalcohol or a diol.

Preferably, the alcohol comprises at least 8 carbon atoms, more preferably at least 18 carbon atoms. In particular, it is selected from the group consisting of n-octan-1-ol, 2-ethylhexanol, 7-methyl-octan-1-ol, nonanol (or nonan-1-ol or pelargonic alcohol), decan-1-ol, dodecan-1-ol, octadecan-1-ol, 1,6-hexanediol, 2-butyl-2-ethyl-1,3-propanediol and diol dimers.

By "dimer (s) diol" is meant a diol dimer or a mixture of diol dimers capable of being obtained by intermolecular dimerization of one or more unsaturated carboxylic acid (s), followed by a reduction reducing acid functions to alcohol functions.

Each unsaturated carboxylic acid may have one or more unsaturations. Preferably, the unsaturation is a double bond.

Each unsaturated carboxylic acid preferably comprises at least 16, more preferably at least 18 carbon atoms.

Advantageously, the unsaturated carboxylic acid is an unsaturated fatty acid derived from a renewable substance, such as vegetable oils, animal oils and animal fats. In particular, the unsaturated carboxylic acid is an unsaturated fatty acid having from 16 to 24 carbon atoms, preferably from 18 to 22 carbon atoms. More particularly, the fatty acid is oleic acid, linoleic acid or a mixture of fatty acids of a vegetable oil such as rapeseed oil, castor oil, sunflower oil, soybean oil, linseed oil or safflower oil.

According to a first embodiment, the dimer (s) diol is a diol dimer. It can be obtained by intermolecular dimerization of an unsaturated carboxylic acid, in particular an unsaturated fatty acid.

Thus, preferably, the diol dimer has from 32 to 48 carbon atoms, more preferably from 36 to 44 carbon atoms, more preferably still, the diol dimer has 36 carbon atoms.

Generally, a diol dimer is a mixture of isomers each of which can have an acyclic or cyclic structure.

Preferably, the diol dimer comprises more than 50% by weight, more preferably more than 70% by weight of acyclic isomer (s), the percentages by mass being given on the total mass of diol dimer.

According to a second embodiment, the dimer (s) diol is a mixture of diol dimers capable of being obtained by intermolecular dimerization of several (at least two, preferably at least three) unsaturated carboxylic acids, in particular unsaturated fatty acids. More particularly, the dimer (s) diol is a mixture of diol dimers each of which may independently comprise from 32 to 48 carbon atoms, preferably from 36 to 44 carbon atoms.

In particular, the mixture of diol dimers is obtained from fatty acids of a vegetable oil selected from the group consisting of rapeseed oil, sunflower oil, soybean oil, linseed oil and safflower oil.

As previously indicated, each diol dimer present in the diol dimer mixture may be in the form of different isomers.

Preferably, the mixture of diol dimers comprises more than 50% by weight, more preferably more than 70% by weight, of acyclic isomer (s), the percentages by mass being given on the total mass of diol dimers.

Advantageously, at least 50%, preferably at least 60% by weight of dimer (s) diol on the total mass of dimer (s) diol has 36 carbon atoms and more than 50% by weight, preferably more than 70% by weight of dimer (s) diol, on the total mass of diol dimers, is acyclic. By total mass of dimer (s) diol is meant the total mass of dimer diol molecules.

Advantageously, the dimer (s) diol is saturated.

Alternatively, the lipoic acid esters are capable of being obtained by transesterification.

The transesterification can be carried out according to any known method, chemically or enzymatically. The transesterification may for example be carried out by heating the reagents, lipoic acid and, triglyceride or carboxylic acid ester other than a lipoic acid ester, to a temperature of at least 90 ° C, in the presence or absence of catalyst. . Transesterification can also be carried out using an enzyme, such as Novozym 435, by heating to 60 ° C.

Triglycerides are advantageously derived from renewable resources, such as oils and fats of plant and animal origin.

For example, triglycerides can come from rapeseed, sunflower, soybean, safflower, flax and tallow oils.

The carboxylic acid esters comprise one or more ester function (s), preferably from one to three, more preferably one or two.

Advantageously, the carboxylic acid esters are esters of fatty acids.

Preferably, in the carboxylic acid esters, the carboxylic acid has at least 8 carbon atoms, more preferably at least 18 carbon atoms.

In particular, the carboxylic acid ester is chosen from the group consisting of caprylic acid esters, capric acid esters, lauric acid esters, myristic acid esters and acid esters. palmitic acid, stearic acid esters, oleic acid esters, linoleic acid esters, linolenic acid esters and mixtures thereof.

Preferably, the carboxylic acid ester is a methyl ester or an ethyl ester.

Whatever the process for preparing the lipoic acid ester, advantageously, all the hydroxyl functions present on the lipoic acid ester are esterified.

As a result of the process for preparing the lipoic acid ester, there may be added one or more ester purification step (s). The purification can be done by any separation method known to those skilled in the art.

Advantageously, the purification of the lipoic acid ester (s) can comprise one or more of the following methods: filtration, evaporation, liquid / liquid extraction and / or the passage over a mineral substrate or a resin such as only silica.

Preferably, the lipoic acid ester is a monoester, a diester, and / or a triester of lipoic acid, more preferably a monoester and / or a diester. In particular, the lipoic acid ester is preferably a monolipoate, a dilipoate and / or a trilipoate, more preferably a monolipoate and / or a dilipoate.

More particularly, the lipoic acid ester is an ester or a mixture of lipoic acid esters represented by the formula (I) in which n is between 1 and 3, preferably equal to 1 or 2.

In particular, the monoester is selected from the group consisting of n-octyl lipoate, 2-ethylhexyl lipoate, 7-methyloctyl lipoate, n-nonanyl lipoate, decyl lipoate, dodecyl lipoate, tetradecanyl lipoate, hexadecanyl lipoate, octadecanyl lipoate, dioleylglycerol lipoate.

The diester is selected from the group consisting of the esters of lipoic acid and 1,6-hexanediol, the esters of lipoic acid and 2-butyl-2-ethyl-1,3-propanediol, the acid esters. lipoic and oleylglycerol, and lipoic acid esters and diol dimers.

According to the UIPAC nomenclature, the name of an ester R-COOR ', is based on the name of the corresponding carboxylic acid (R-COOH) whose ending "ique" becomes "ate", and on the name of the alkyl group (R ') which ends with "yle".

However, sometimes the name of the ester is formed by the names of the reagents, such as acids and alcohols, used in the process for preparing the ester.

A third nomenclature used is to use the name of the carboxylic acid in "ate" and the name of the alcohol as it is.

Thus, n-octyl lipoate, lipoic acid and n-octanol ester and n-octanol lipoate refer to the same compound.

The invention also relates to a method for improving the anti-wear and / or friction-modifying property of a mineral oil by adding a lipoic acid ester.

The invention also relates to a composition comprising a lipoic acid ester and a mineral oil. The lipoic acid ester is as described above, including the preferred and advantageous modes.

The mineral oil is advantageously a group I, II and / or III mineral oil as described above.

Group II mineral oils are widely used in the metalworking industry. Also, advantageously, the oil of the composition according to the invention is a Group II mineral oil.

Preferably, the composition according to the invention comprises a total amount of lipoic acid ester (s) of at least 2% by weight, more preferably at least 5% by weight, more preferably still at least 10% by weight. relative to the mass of the composition.

The skilled person knows how to choose the lipoic acid ester (s) best adapted (s) according to the chosen application and the appropriate amount.

Preferably, the composition according to the invention also comprises an additive other than a lipoic acid ester.

Advantageously, the composition according to the invention is a lubricating composition.

Preferably, the lubricating composition is a metal working oil, a motor oil, a transmission oil, a hydraulic oil, or a gear oil.

In the field of lubricants, the oil represents at least 50% by weight, preferably at least 70% by weight, still more preferably at least 85% by weight of the mass of the lubricating composition.

Advantageously, the additive other than a lipoic acid ester is chosen from the additives used in the field of lubricants, such as antioxidant additives, anti-wear additives, viscosity index improvers, additives friction modifiers, extreme pressure additives, pour point depressants, anti-foaming additives, demulsifying additives, corrosion inhibiting additives, thickening additives, detergent additives and dispersant additives.

The skilled person knows how to choose the additive (s) best suited (s) depending on the chosen application, such as a lubricant application.

The additive other than a lipoic acid ester may be chosen from:

antioxidant additives, such as dialkyl dithiophosphates, substituted phenols and / or aromatic amines; anti-wear additives, such as zinc dialkyl-dithiophosphates and / or phosphorus derivatives, which can be added in a quantity less than that usually used;

viscosity index improvers, such as polymers of olefin copolymer type (OCP), polyisobutenes, polymethacrylates, diene polymers, polyalkylstyrenes and / or molybdenum derivatives;

- Friction modifying additives other than a lipoic acid ester, such as glycerol monooleate (GMO), they may be added in an amount less than that usually used;

extreme pressure additives, such as organometallic molybdenum derivatives, fatty acid derivative compounds, phosphosulphurized molecules and / or borates;

pour point depressant additives, such as metal soaps, carboxylic acids, polymethacrylates, alkylphenols, dialkylaryl esters of phthalic acid, maleate-styrene copolymers, naphthalene paraffins and / or polyesters of vinyl fumarate acetate type;

anti-foam additives, such as silicone oils, silicone polymers and / or alkyl acrylates;

demulsifying additives, such as propylene oxide copolymers; anticorrosive (or anti-rust) additives such as alkali metal and / or alkaline earth metal sulfonates (Na, Mg, Ca salts), fatty acids, fatty amines, alkenyl succinic acids and / or their derivatives, benzotriazole, and / or tolyltriazole;

thickening additives, such as fatty esters;

detergent additives, such as the calcium and / or magnesium salts of alkylarylsulfonates, alkylphenates, alkylsalicylates and / or their derivatives;

dispersant additives, such as alkenylsuccinimides, succinic esters and / or their derivatives, and / or Mannich bases;

metal deactivating additives, such as heterocyclic compounds containing nitrogen and / or sulfur, for example triazole, tolutriazole, and benzotriazole;

or their mixtures.

It should be noted that an additive may have several properties, such as zinc dialkyl dithiophosphate, which is an antioxidant, anti-wear, anti-corrosive and slightly dispersing additive. Because of the anti-wear, friction modifying, viscosity index, antioxidant and extreme pressure properties of the lipoic acid esters, the presence of one of these esters in the composition makes it possible to reduce the amount usually necessary additive other than a lipoic acid ester, thus reducing the environmental impact of this other additive, especially if it has some toxicity.

In particular, the amount of the additive other than a lipoic acid ester selected from the group consisting of anti-wear additives, friction modifying additives, viscosity index improvers, anti-slip additives and the like. The oxidant and the extreme pressure additives may be adapted depending on the amount of lipoic acid ester (s) present in the composition according to the invention.

The invention also relates to a process for preparing a composition according to the invention, comprising a step of mixing a mineral oil with a lipoic acid ester.

The mineral oil and the ester are as described above, including advantageous and preferential modes.

The compositions according to the invention are particularly suitable for use as a lubricant composition and / or in the field of the petroleum industry.

The invention will be better understood from the following examples, given by way of illustration, with reference to:

Figure 1, which is a graph showing Stribeck curves (i.e. curves representing friction coefficient versus rotational speed measurements) obtained from non-additive mineral oil and the same oil additive of 10% by mass of diol dimeric dilipoate (1 test and 2 repetitions with each sample); Figure 2, which is a graph showing the average improvement (on the test and the two repetitions of Fig. 1) of the coefficient of friction between a non-additive mineral oil and the same oil additive of 10% by mass of dilipoate of diol dimers;

FIG. 3, which is a graph showing the average improvement (on the test and the two repetitions of FIG. 1) of the coefficient of friction between a non-additive mineral oil and the same oil with 7% by weight of lipoate additive; n-octyl;

- Figure 4, which is a graph showing Stribeck curves obtained from a non-additive mineral oil, the same oil additive of 10% by weight of diol dimeric dilipoate, the same oil additive of 7% by weight of n-octyl lipoate and the same oil additive of 10% by weight of GMO.

Example 1 Preparation of Lipoic Acid Esters

1 .1 -Preparation of a dilipoate of diol dimers

100 g of lipoic acid (lipoic acid, 98%, CAS No. 1077-28-7) and 132.5 g of diol dimers (Radianol® 1990 marketed) are added to a reactor equipped with a distillation assembly. by Oleon NV, composed of at least 98% diol dimers, at least 95% diol dimers having 36 carbon atoms, and at least 75% acyclic isomers).

The mixture is heated to 140 ° C with stirring. The progress of the reaction is monitored by thin layer chromatography. After 24 hours of reaction, the conversion of the alcohol is complete.

The reaction mixture is purified on Combi-Flash by passage through a column of silica gel, eluting with heptane with a gradient of 0 to 5% acetone.

After evaporation of the eluent, the diol dimeric dilipoate is obtained in the form of a yellowish viscous oil.

The diol dimeric dilipoate mixture obtained contains 98.3% diol dimeric dilipoate having 52 carbon atoms and 76% acyclic isomers.

1 .2-Preparation of n-octyl lipoate

The preparation uses lipoic acid (lipoic acid, 98%, CAS no: 1077-28-7) and n-octan-1 -ol from Merck, according to the preparation method described in the publication "Lipoate ester multifunctional" lubricant additives "; Biresaw et al .; Industrial & Engineering Chemistry Research (2016), 55, 373-383. Example 2 Preparation of Samples of Compositions According to the Invention and Comparative Samples

Samples were prepared using as raw materials: Group II Chevron Neutral oil 220R mineral oil (hereinafter referred to as "mineral oil"),

the dilipoate of diol dimers of Example 1, the n-octyl lipoate of Example 1,

the anti-wear additive zinc dialkyldithiophosphate (hereinafter referred to as "ZDDP"), and

Oleon Radiasurf® 7149 glycerol monooleate friction modifier additive (hereinafter referred to as "GMO").

The content of the samples is shown in Table 1 below:

^* on the total mass of the sample

Table 1: Content of samples tested

The composition samples and the comparative samples are obtained by simple mixing, that is to say bringing the compounds into contact and manual stirring. Example 3 Evaluation of the anti-wear property of the lipoic acid esters using the 4-bead test

The anti-wear property of the lipoic acid esters prepared in Example 1 was evaluated with the four-ball wear test according to ASTM D4172-94 (2016). To do this, the samples prepared in Example 2 were tested. The test consists in applying three balls disposed on the same plane and immersed in a sample, a weight of 40 kg with rotation at 1200 rpm for one hour at 75 ^ via the ^4th ball placed at the intersection of three other marbles.

The test results are summarized in Table 2 below:

Sample wear (mm) gain on wear (%) mineral oil 1, 1 10

Composition C 0.625 -43.7% Composition E 0.650 -41, 4%

Composition I 0.775 -30.2%

Composition K 0.783 -29.5%

Table 2: Four Ball Wear Test Results

The addition of lipoic acid ester in the mineral oil makes it possible to reduce the wear of the metal in contact with the compositions according to the invention. In particular, the addition of 2% by weight of diol dimeric dilipoate in a Group II mineral oil makes it possible to reduce the wear of the metal by more than 41%.

Example 4 Evaluation of Friction Modifying Properties of Lipoic Acid Esters

To quantify the decrease in friction, the coefficient of friction of the samples prepared in Example 2 (mineral oil, compositions C, E and I) was measured on an MCR-502 rheometer equipped with an MCR tribometer. Anton-Paar.

A 100Cr6 steel ball is brought into contact with 3 cylinders made of 100Cr6 steel with a rotation ranging from 1000 mm / s to 0.01 mm / s at 25 ° C and applying a force of 10 Newtons. The three cylinders and the ball are immersed in 0.5 mL of the test sample. The coefficient of friction as a function of the rotational speed is then measured to establish a Stribeck curve.

With mineral oil (not additive), it has been observed that by repeating the test twice, the coefficient of friction at low speed (<0.25 mm / s) increases substantially each time (Fig. 1). .

On the other hand, with composition C, the coefficient of friction remains stable after 2 repetitions, at values much lower than those obtained with mineral oil alone (FIG.

At low speeds and thus maximum metal-to-metal contact, the coefficient of friction down to 53% is observed with composition C (FIG 2).

The same results are obtained with composition E, lowering the loading from 10% to 2% by weight of diol dimeric dilipoate in mineral oil.

Similarly, at low speeds and thus maximum metal-metal contact, a reduction in the coefficient of friction of up to 44% is observed with composition I (FIG 3).

The addition of 7% by weight of n-octyl lipoate or 2% by weight of diol dimeric dilipoate or in a Group II mineral oil reduces the friction respectively 44 or 53% compared to a non-additive oil use. The lipoic acid esters can therefore be used as friction modifier additives. Example 5 Evaluation of the Anti-Wear and Friction Modifying Properties of Lipoic Acid Esters Using the HFRR Test

The anti-wear and friction modifying properties of the lipoic acid esters prepared in Example 1 were evaluated via the High Frequency Reciprocating Rig (HFRR) test, a high frequency reciprocating apparatus, similarly to what is described in ASTM D6079-1 (2016).

To do this, the samples (mineral oil, compositions C and K) prepared in Example 2 were tested.

The test involves applying a weight to a moving ball on a disc to measure wear and coefficient of friction at different temperatures. A few milliliters of each sample are used to immerse the ball before each test.

With the composition C, the wear of the ball is lowered to 153.9 x 156.6 is 40% x 32% decrease. There is also a decrease in the friction coefficient of 14% to 70 ^<C, 44% at 100 ^<C and 54% to 130 ^<C.

With the composition K a decrease in the wear of the ball is also observed, a decrease of 16% x 24%. The coefficient of friction is also reduced by 14% to 70 ^<C, 38% at 100 ° C and 41% at 130 ° C.

This HFRR test confirms the anti-wear property of the lipoic acid esters and also shows their property modifying friction.

Example 6: Comparative - Four ball wear test with ZDDP antiwear additive

The comparative test was carried out under the same conditions as those described in Example 3. The test carried out with Comparative 1 prepared in Example 2 leads to a wear of 0.570 mm, ie a decrease in wear of 48.6%.

The diol dimer dilipoate has an anti-wear property (43.7% decrease) comparable to the ZDDP, which is one of the most used anti-wear additives.

Example 7 Comparative Rheometer Test with the GMO Friction Modifier Additive

The test with Comparative 2 prepared in Example 2 was carried out under the same conditions as those described in Example 4.

This test shows that the lipoic acid esters lower the coefficient of friction of the mineral oil, similarly to the GMO, especially in the case of the diol dimer dilipoate (FIG 4).

Claims

1. Use of a lipoic acid ester as an antiwear additive and / or friction modifier for mineral oil.

2. Use of a lipoic acid ester according to claim 1, and as an extreme pressure additive, antioxidant additive and / or viscosity index improving additive.

3. Use according to claim 1 or 2, wherein the lipoic acid ester has at least 16 carbon atoms.

The use according to any one of claims 1 to 3, wherein the lipoic acid ester is a monoester, a diester and / or a triester of lipoic acid.

5. Composition comprising a lipoic acid ester and a mineral oil.

6. Composition according to claim 5, comprising a total amount of lipoic acid ester (s) of at least 2% by weight.

7. Composition according to claim 5 or 6, comprising at least 50% by weight of mineral oil.

The composition of any one of claims 5 to 7, wherein the mineral oil is a Group II mineral oil.

9. Composition according to any one of claims 5 to 8, further comprising an additive other than a lipoic acid ester.

10. Process for the preparation of a composition according to any one of claims 5 to 9, comprising a step of mixing a mineral oil with a lipoic acid ester.

1 1. Use of the composition according to any one of claims 5 to 9 as a lubricating composition.